Today's optical communication systems include, among others, high data rate systems (i.e., speeds at 10 Gb/s and above), long-haul systems, and metropolitan SONET systems. In most current applications, these systems utilize single mode fiber (SMF) as the transmission medium, which limits the link length to at most 80 km, primarily as a result of impairments in the fibers themselves. Moreover, there exist many “legacy” systems that include multimode fiber (MMF) as their backbone communication link. Again, these systems are limited in their link length to no more than 26 m as a result of dispersion problems along the fiber, particularly noticeable at the higher data rates.
More particularly, “impairment” in optical transmission is known to result from various types of optical pulse dispersion along the transmission fiber, where the three main types of dispersion are chromatic dispersion, modal dispersion and polarization mode dispersion. Chromatic dispersion, the result of changes in the physical properties of the fiber itself, is defined as the spreading (in wavelength) of a pulse of light as it propagates over great distances. The longer the fiber over which the pulse travels, the wider the pulse will spread. Difficulties arise when the resulting energy from a pulse begins to interfere with the energy in an adjacent pulse. This interference causes intersymbol interference (ISI) in the electrical domain. The spreading of symbols across each other causes errors; the receive side of the link cannot easily distinguish a “1” from a “0”, since they are no longer at the ideal logic levels. Depending on the fiber, pulse spreading may cross over several unit intervals (UIs), with a dispersion of “one UI” defined as immediately adjacent symbols are interfering with each other.
Interference between “modes” of light arriving at a receiver at different times causes modal dispersion in multimode fibers. In particular, for data rates of 10 Gb/s, the ISI resulting from modal dispersion is approximately 5 UI for 220 meters of multimode fiber.
Polarization mode dispersion (typically a concern of SMF applications) is a phenomenon in which a single pulse appears as multiple pulses farther down the fiber. A “perfect” optical fiber would allow for two, orthogonally polarized modes to travel indefinitely along an expanse of fiber. However, various factors (including fiber bends, stress and the like) result in one polarization mode propagating at a different speed than the other, causing a phase difference in their arrival at a receiver.
A variety of techniques have been offered in the past to address these dispersion problems as outlined above. For many years, a common approach was to implement dispersion-shifted fiber, where the fiber was manufactured to exhibit minimal dispersion problems at the common wavelength(s) used for optical communication systems. Another approach was to use dispersion-compensated fiber—again, an arrangement where the fiber was particularly configured to “pre-distort” a propagating optical signal through manipulation of the characteristics of the fiber itself. While these techniques are useful in many situations, the ability to distribute new types of fiber cables in “legacy” systems is not always an option.
There have also been various optically-based arrangements for “measuring” an amount of dispersion present in a received optical signal, and then providing a type of corrective optical signal to essentially compensate for the dispersion. U.S. Pat. No. 6,498,886 issued to D. J. Sobiski et al. on Dec. 24, 2002, discloses an optical dispersion compensation arrangement that is configured as a feedback control module within an optical receiver. The dispersion (both chromatic and polarization mode) are measured in the signal and a “correction” signal is applied if the measured dispersion is above a predetermined threshold. The system of Sobiski et al. is considered to be “adaptive” since measurements are continually made on the arriving optical signal and adjustments are made in real time.
While the Sobiski et al. arrangement—and other conventional optical dispersion compensators—are useful in providing a degree of dispersion compensation, problems begin to manifest themselves when the data rate of the system approaches 10 Gb/s and beyond. That is, a purely optical compensation arrangement cannot “cope” with extreme data rates and the compensation begins to lag the signal speed.
A relatively new type of dispersion compensation that addresses this speed issue is defined as “electronic dispersion compensation” (EDC); in an EDC arrangement, an incoming optical signal is first transformed into an electrical signal. The compensation is then applied to the electrical signal and the “compensated” signal is then re-converted (if necessary) into an optical signal. Since the various types of CMOS-based circuits used to provide the necessary compensation can easily handle data rates in the 10-100 Gb/s range, an EDC arrangement can easily cope with optical communication system speeds.
To date, EDC has been implemented using a variety of equalization algorithms. The three most common algorithms may be classified as: “continuous time filters” (CTF), “feedforward-equalizer/decision-feedback-equalizers” (FFE/DFE), and “maximum-likelihood-estimator equalizers” (MLSE). CTFs offer the simplest, most cost-effective and lowest power consumption compensation alternative, but are known to also remove a portion of high frequency signal components. FFE/DFE algorithms apply a more sophisticated approach to compensation, using a multi-tap algorithm to compensate for ISI that exceeds one UI of interference. A common implementation of an FFE/DFE arrangement generally comprises an automatic gain control block, a CTF/FFE block, a DFE block, a clock/data recovery (CDR) block, and a least-mean-squared (LMS) adaptation block.
As mentioned above, adaptability is an essential characteristic for EDC arrangements. As optical fibers degrade over time and/or introduce new sources of interference, the EDC arrangement must be able to recognize the changes and adapt the applied compensation. These new sources of interference include, but are not limited to, fiber kinks, fiber bend radius change associated with mechanical vibration (for example, with air flow from cooling fans), ambient temperature changes and the like. All of these factors need to be recognized and the EDC must be adapted to refine the algorithm used to control the applied compensation. Self-adaptation requires closed-loop-feedback mechanisms that enable equipment to calibrate itself by slightly modifying filters and gains that improve signal response until the system achieves an ideal signal.
In many new and expanding applications, a multiple number of fibers are used to establish communication along an optical link from one point to another. Examples of such applications are multi-wavelength WDM systems, or “parallel optical devices” (PODs). Typically, these links employ 2, 4 or 12 fibers (also referred to as “channels”). Typically, each channel carries data that is uncorrelated with the data carried along the other channel(s). As a result, each channel requires its own EDC, independently correcting for fiber-specific dispersion problems, increasing the cost and complexity of a multi-channel optical receiver.